1. Field of the Invention
The present invention relates to a fuel cell, and more particularly to a passive fuel cell assembly.
2. Description of Related Art
With the rapid development of industry, the consumption of conventional energy source such as coal, petroleum, and natural gas is increasingly high, and due to the limited storage of natural energy source, novel alternative energy source may be researched and developed to substitute the conventional energy source, and the fuel cell is taken as an important and practical choice.
In brief, the fuel cell is substantially a power generator that converts chemical energy into electric energy by utilizing the reverse reaction of the water electrolysis. The proton exchanging membrane fuel cell mainly includes a membrane electrode assembly (MEA) and two electrode plates. The MEA includes a proton exchange membrane, an anode catalyst layer, a cathode catalyst layer, an anode gas diffusion layer (GDL) and a cathode GDL. The anode catalyst layer and the cathode catalyst layer are respectively disposed on two sides of the proton conducting membrane, and the anode GDL and the cathode GDL are respectively disposed on the anode catalyst layer and the cathode catalyst layer. Furthermore, two electrode plates include an anode and a cathode, which are respectively disposed on the anode GDL and the cathode GDL.
Currently, the common proton exchanging membrane fuel cell is Direct Methanol Fuel Cell (DMFC), which directly takes the methanol aqueous solution as the source for supplying fuel, and generates currents through the relevant electrode reaction between methanol and oxygen. The reaction formulas of the DMFC are shown as follows:Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
During the reaction, 1 mol of water is consumed at anode, three mol of water is generated at the cathode, and the water generated in the reaction may be removed immediately, and cannot be kept at the surface of the catalyst layer, in such a manner, the fuel cell can react continuously, thereby generating currents.
In recent years, passive type fuel transmission in fuel cells is developed, wherein fuel transmission and water management are achieved spontaneously by films designed on the outside of MEA. Since stability of fuel cells is significantly affected by the water management therein. As for the water management in fuel cells, many solutions have been proposed in this field. For instance, in U.S. Publication No. 2004/0209136, a water management layer is used to prevent moisture leaking from an anode. The water management layer includes at least one porous layer mainly made of polytetrafluoroethylene (PTFE). The porous layer allows gaseous fuel to pass through and obstructs water.
In the fuel cell discussed in WO 2005/112172A1, liquid methanol fuel is vaporized and a moisture-maintaining layer is used to prevent the liquid methanol fuel from being diluted. In order to prevent the liquid methanol fuel from being obstructed by the moisture-maintaining layer, the moisture-maintaining layer is defined as followings. According standard of JIS K7126-1987A, gaseous methanol permeability of the moisture-maintaining layer is about 1×105˜1×109 cm3/m2·24 hr·atm. However, stability of the fuel cell discussed in WO 2005/112172A1 still deteriorates and inner electrical resistance of the fuel cell increases during a long time operation.
As for discharge design of carbon dioxide in fuel cells, WO 2006/040961 and WO 2006/040961 have proposed their own solutions. During a long time operation, inner electrical resistance in fuel cells proposed by WO 2005/112172A1, WO 2006/04096, or JP 2006-134808 still increases, since water accumulation phenomenon can not be improved. Additionally, since the water management layer of U.S. Publication No. 2004/0209136 and the fuel gas diffusion layer of JP 2006-134808 are hydrophobic and gas-permeable, the distribution of gaseous fuel is not uniform when fuel cell is inclined during operation. The same problem occurs in the moisture-maintaining layer of WO 2005/112172A1. Accordingly, how to improve uniformity of gaseous fuel transmitted to the anode catalyst layer, water accumulation phenomenon at the anode, and stability of fuel cells during a long time operation become crucial issues when fabricating fuel cells.